<script>
function a(){
var slt=document.getElementById("abc");’取得标签select
if(slt.value==""){ '判断是否选中
alert("选择一项");
return false;
}
return true;
}
</script>
<form method=post onsubmit=" return a()">
<select id="abc">
<option value="">选一项</option>
<option value="x">A</option>
<option value="z">B</option>
</select>
<input type=submit value="提交">
</form>
